Why do globular clusters exist?

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What causes old stars to bunch up like globular clusters? It cannot just be gravity alone, can it?

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What do you mean by “can’t be gravity alone”? That’s the only real attractive force on cosmic scales.

And you’re getting the timing wrong. The gas bunched up first, then formed stars. It’s believed that they form early in a galaxy’s life, when there’s lots of free gas, which is spread too thinly to completely collapse, but still close enough to become gravitationally bound together.

The stars in globular clusters are not old. In fact the average age seams to be lower in globular clusters then elsewhere in the galaxy. It seams like globular clusters are formed when dust and gas gets attracted to each other by gravity and then forms into dense clouds. Parts of these clouds are dense enough that gravity takes over completely and it forms into stars. So the stars do not bunch up into a globular cluster but instead they are formed in the cluster.

It’s not that old stars are bunching up in globular clusters, but that the clusters themselves are old. Globular clusters formed many billions of years ago and very quickly ran out of gas for star formation. This means that the only stars left in them are either very old, or were formed by stellar collisions which are very rare.

The exact mechanism which forms globular clusters is poorly understood. We know it’s definitely to do with gravity, and is related to the formation or interaction of galaxies. But an exact mechanism for formation of them all is not known.